From the various types of modulation used for optical signals, FSK ("Frequency Shift Keying" modulation is used extensively. It consists of moving the frequency of the optical signal emitted according to the information to be transmitted. This modulation is easily obtained with a monofrequency semiconductor laser by modulating the polarization current. Contrary to the case with intensity modulation, all the photons emitted by the laser are frequency-modulated, regardless of the modulation depth. In other words, it is equivalent to a 100% modulation.
The detection of FSK modulated optical signals is generally effected by either coherent dectection or by direct intensity modulation conversion via the Fabry-Perot standard. The first method requires the use of a local oscillator (generally constituted by a wavelength tuneable monofrequency laser), a fast photodetector, a signal mixer and complex electronic circuits (intermediate frequency discriminator, decision circuit, intermediate frequency stabilization circuit, etc); if this results in obtaining good performances, it nevertheless remains difficult to implement. The second method is scarcely less so, requiring as it does the use of a control circuit to tune one of the resonance frequencies of the standard with that of the signal to be demodulated. In addition, it does not make it possible to tune the frequency over a wide range, this proving to be incompatible with the high density wavelength multiplexing of channels.
As regards these problems concerning optical telecommunications modulation and demodulation, reference may be made to the article by Yoshihisa YAMAMOTO et al published in the IEEE Journal of Quantum Electronics, vol. QE-17, No. 6, June 1981, pp. 919 to 935.
Furthermore, there are a number of photoreceptors relatively simple to implement which use a semiconductor laser into which the luminous beam to be processed is injected. These devices do not strictly function as demodulators, but rather as automatic frequency control means. Such a device is described in the article by Soichi KOBAYASHI et al and entitled "Automatic Frequency Control in a Semiconductor Laser and an Optical Amplifier" published in the Journal of Lightwave Techology, vol. LT-1, No. 2, June 1983, pp. 294-401 and in the article by Soichi KOBAYASHI et al and entitled "Injection Locking Characteristics of an AlGaAs Semiconductor Laser" published in the IEEE Journal of Quantum Electronics, vol. QE-16, No. 9, September 1980, pp. 915 to 917.
As the invention to be described shortly makes use of certain elements of this known technique, the main characteristics of the latter shall not be described again.
The technique described in the afore-mentioned articles uses a semiconductor laser into which the luminous beam to be processed is injected ("Optical-Injection-Locking" technique). The laser is fed with current approaching the threshold (that is, virtually between 0.7 and 1.3 times the threshold current). The laser then functions as an amplifier system and is referred to as an "injection-locked amplifier" (ILA) or a "resonant-type amplifier" (RTA). The laser emitting the radiation is known as the "master" laser and the laser into which the radiation is injected is known as the "slave" laser.
The slave laser operates within a certain frequency band. At the crossing of the active layer of the amplifier, the injected beam is amplified according to the place occupied by the frequency of the beam within the amplification band. This amplification is maximum at the center of the band and decreases on both sides.
The injection of photons into the active layer has the effect of significantly increasing the density of photons and correlatively the optical power produced. As a result, the voltage at the terminals of the laser reduces slightly. As indicated on the curve of the accompanying FIG. 1 (which approximately reproduces one of the curves of FIG. 6 of the first article by KOBAYASHI referred to earlier), the reduction of the voltage V has a resonant shape (that is, an inverted bell) with a minimum (or, if desired, a voltage difference maximum) when the frequency F of the injected optical signal deriving from the master laser is equal to the central frequency Fo of the amplification band of the slave laser. This voltage difference reduces when the frequency F is removed from Fo so as to be annuled when the frequency F leaves the locking zone.
This phenomenon is naturally linked to the value of the polarization current of the slave laser. FIG. 2 (which roughly reproduces one of the curves of FIG. 4 of the aforesaid document) shows the variation of the voltage .DELTA.V according to the ratio I/Io. This variation is that much larger when the polarization current approaches the threshold current (I/Is=1). The voltage variation becomes almost nil when the slave laser is polarized by a current 1.3 or 1.4 times greater than the threshold current. (The logarithmic scale is to be noted in ordinates). By extrapolating the threshold twice, a voltage variation of about 1/100 millivolts would be found and unable to be exploited. This is why in this technique, one is always close to the threshold (most of the measurements referred to in the article mentioned correspond to a polarization current equal to 1.1 times the threshold current).
It can be conceived that a voltage variation such as that of FIG. 1 may make it possible to carry out frequency control: if the frequency F of the master laser is taken as a reference, by taking the variation of the voltage at the terminals of the slave laser as an error signal, it could be possible to bring the central frequency Fo of the slave laser back to the frequency F by modifying the temperature of the slave laser, for example.
If on the other hand, it is desired to know the variations of the frequency of the signal injected with respect to the frequency Fo, it would be necessary to derive the voltage variation represented on FIG. 1. Then a curve would be obtained such as the one of FIG. 3, which roughly corresponds to one of the curves of FIG. 12 of the aforesaid document.
This additional derivation operation constitutes a complication. In fact, it is dispensed with since this method remains a means for stabilizing the frequency of a slave laser with respect to a master laser and not a frequency demodulation device.
In this known technique, as mentioned earlier, the semiconductor slave laser functions as an amplifier close to the threshold and the voltage taken at its terminals always undergoes a negative bell-shaped variation, the measured voltage variation only giving the frequency difference but not its sign.